In the past, various methods have been used to measure the flow rates of gases. Using one of these methods, attempts have been made to develop a portable apparatus which accurately measures the flow rate of a gas source through a fixed volume by monitoring the travel time of a liquid soap film moving through an elongated cylindrical tube in response to the flow of the gas through the tube. Numerous devices incorporating this measurement method have been developed in the past, the prior art including a flow standard manufactured by A. P. Buck, Inc., 3139 S. Orange Ave., Orlando, Fla. 32806. Generally, such devices have used a microprocessor in combination with sensing circuits to measure the time for a soap film pushed by the gas flow to travel between a pair of electro-optic sensors placed in a tube. The travel time of the soap film is proportional to the flow rate at which a volume of gas passes between the electro-optic sensors. Thus, the flow rate of the gas is computed based on the volume of the tube and the time for the soap film to travel a predetermined distance. Devices of this general design have achieved accuracy levels of approximately + or -0.5%.
Prior designs have experienced difficulty in developing suitable soap films in flow meters. Typically, more than a single film of soap is formed or a film is formed which is not perpendicular to the tube wall. Other factors which are believed to have limited the success of such devices include the difficulty of providing and maintaining an adequate layer of liquid soap on the inner wall of the tube in order to effect smooth low-friction movement of thin soap films through the tube; the need for a more reliable means of returning the soap film material to a reservoir in the device; the practice of positioning each light emitting device along the measurement tube wall in more or less of a diametrically opposite position with respect to each corresponding light sensing device, thus necessitating a bulky instrument design; and the difficulty of bringing the performance of such devices into acceptable tolerance limits without precision tolerancing of tube dimensions and sensor locations on the tube. Such exacting dimensions have significantly affected manufacturing costs. Furthermore, because such portable devices are needed for field calibration of pumps and inspection of equipment, it is desirable to have a more reliable device of smaller size and weight than available in the prior art yet which device also assures an acceptable level of accuracy.